A walk along the intertidal band at Alki Point presents an intriguing puzzle to the beachcomber. Sifting through discarded mollusk shells littering the shore, allowing a moment’s scrutiny for each, one begins to see a curious pattern emerge. Near the umbones (the bulges on either side of the hinge) of many of the bivalve shells is a small hole, perfectly symmetrical and slightly countersunk, as if the shell had been drilled through by a jeweler stringing a necklace. A number of empty snail shells sport an identical hole on the largest whorl, above the spiral’s mouth. Besides these minor flaws, the shells appear intact and in good shape, betraying no other evidence as to the fate of their inhabitants. Who, or what, is doing this beach-wide precision drilling?
Meet the Lewis’s moon snail (Lunatia lewisii), cold-blooded clam killer of the North Pacific. (Moon snails eat snails, too, including other moon snails—but we’ll keep clam on that.) Largest of the Naticidae, a family of predatory—yes, predatory—sea snails found throughout the world’s oceans, these phlegmatic gastropods are the bane of a bivalve’s existence. They are big and beefy, with shells the size of grapefruits and gobs of pearlescent, gelatinous flesh. Their foot (essentially the snail’s soft, external anatomy) can swell with seawater to four times the shell’s volume, extruding over and around the mantle like an aegis of living snot. The shell all but engulfed by the snail’s bulk, it is hard to believe that, if harassed, the creature can retract fully into its mobile home—but retract it does, albeit very, very slowly. Moon snails glide along on a track of mucus, prowling the sandy substrate in search of prey too haplessly immobile or perilously unaware to thwart their advances.
Moon snails love to eat clams, and like many other clam enthusiasts, they dig up their quarry siphon-first. A snail will burrow beneath the topmost layer of sand, probing with its antennae for the telltale excrescence of a clam neck. Once found, the snail latches on powerfully and envelopes the clam with its copious foot, alternately pulling and contorting its body to displace the surrounding sand and dislodge the bivalve. (One might imagine the victim shrieking in terror at this point, had it the means to do so.) In a death scene worthy of “The Blob”, the snail positions its catch umbo-up and smothers it to death with slime. Soon afterward the drilling begins. (Again, one imagines cinematic sound effects: the bone-chilling suck of the snail’s foot, the victim’s gurgling last breath, the spine-tingling whine of a dentist’s bit. Alas, it is a silent struggle, and neither actor understands vertebrate imagery, anyway.) Using its tongue-like radula like an auger, the snail bores through the clam’s umbo, aided by an acidic secretion that partially dissolves the calcium carbonate shell. At last the snail will eat, using its hollow proboscis to dredge out the clam’s soft tissues in what could perhaps be described as a shellfish protein shake.
As is befitting a villain of such dastardly renown, the moon snail leaves not one calling card but two: the drill hole, of course, as well as its egg-sheltering “sand collar”, often found at low tide and sometimes mistaken for a busted-up toilet plunger. It is a distinctive ring-shaped structure composed solely of mucus and sand, and it harbors the next thousand-strong generation of clam-bushing, drill-toting snail-nailers. Only a handful of these teeming larvae will survive into adulthood, but it is no exaggeration to say that those paltry, lubricous few will leave a trail of pierced shells in their wake.